powder charge weight inconsistent

   Hey guys,when I go shooting I have my powder tubes,bullets and modules loaded in loading blocks that hold 21 of each. Usually I wont fire all 21 loads. Before I shoot again, I will  replenish the ones that were fired. Sometimes a month or so later. For some reason the powder weights  change. Loads that were 73.5 grs of 4198 are now 74grs. a month later. Since the tubes have screw tops I would think they are air tight. So it seems that my rcbs balance beam scale (m500) might be causing the problem.( scale is kept dust free and clean)  When I weight charges and then re-weight them again they are dead accurate.
  I know .5 gr is not much, but I like to keep things consistent. Has anyone else had this issue? Thanks in advance.     Leftyml